The present invention concerns an electron-optical system for use in performing observations or inspections, etc., of sample surfaces by means of an electron beam, and an inspection method using the same.
Electron microscopes using electron beams have been widely used in the past for the observation and inspection of miniaturized, highly integrated semiconductor elements, etc. Electron microscopes include microscopes known as low-energy electron microscopes (K. Tsuno, Ultramicroscopy 55 (1994) 127-140 xe2x80x9cSimulation of a Wien filter as a beam separator in a low energy electron microscopexe2x80x9d).
A low-energy electron microscope will be briefly described with reference to FIG. 3. An electron beam (irradiating electron beam S) which is accelerated to approximately 10 keV by an electron gun 1 is shaped by illumination lenses 2 and 3, and is then directed onto a beam separator 4. The electron beam deflected by the beam separator 4 passes through an aperture diaphragm 5, and then irradiates a sample 7 after passing through a cathode lens 6.
Here, the cathode lens 6 consists of three electrodes 6a, 6b and 6c installed along to the direction of the optical axis, and is an electron lens, i. e., a so-called einzel lens, in which the center electrode 6b of the above-mentioned electrodes is biased to a negative potential, and the electrodes 6a and 6c on both ends are grounded. The potential of this central electrode 6b is a high potential of approximately xe2x88x927 to xe2x88x9210 kV.
Meanwhile, the sample 7 is biased to a high voltage of approximately xe2x88x9210 kV, so that an electric field is formed between the sample 7 and the first electrode 6a of the cathode lens 6 which is positioned closest to the sample 7. The irradiating electron beam S reaches the sample 7 after being decelerated to approximately 10 eV by this electric field.
When the sample 7 is irradiated by the electron beam, secondary electrons, reflected electrons and back-scattered electrons, etc., are emitted from the sample. Electrons of at least one of these types constitute an observational electron beam K. Here, the velocity of the reflected electrons is approximately 10 eV.
The observational electron beam K that is emitted from the sample 7 is again accelerated to approximately 10 keV by the electric field formed between the sample 7 and the first electrode 6a of the cathode lens 6. Afterward, the observational electron beam K passes through the other electrodes 6b and 6c of the cathode lens 6, and then enters the beam separator 4 after further passing through the aperture diaphragm 5. Then, the observational electron beam K, which passes through the beam separator 4 in a straight line as a result of the Wien condition being satisfied, is focused as an image on an electron beam detector 11 such as an MCP (micro-channel plate), etc., after passing through image-focusing lenses 8 and 10.
Here, like the cathode lens 6, the illumination lenses 2 and 3 and image-focusing lenses 8 and 10 are einzel lenses, and the central electrodes of these lenses are biased to a high potential of approximately xe2x88x925 to xe2x88x9210 kV.
The above-mentioned conventional electron-optical system possesses the following advantage: specifically, since the energy of the electron beam is high when the electron beam passes through the illumination lenses, cathode lens and image-focusing lenses, the chromatic aberration is low. However, the following two major problems have been encountered:
The first problem is that the cost of the electron-optical system is extremely high. Specifically, as was described above, the einzel lenses constituting the illumination lenses, cathode lens and image-focusing lenses all require the application of a high voltage. As a result, extremely expensive high-voltage power supplies and electrodes with a high withstand voltage are used.
Here, in cases where a relatively low voltage is applied to the respective einzel lenses instead of a high voltage being applied, i. e., in cases where a low-cost power supply and electrodes are used, the focal lengths of the respective einzel lenses are increased, so that the overall length of the electron path is increased by a corresponding amount. Thus, since the size of the electron-optical system is increased, it is difficult to reduce the cost of the einzel lenses.
The other problem is that a long time is required for the elevation of the voltage that accompanies the application of a high voltage to the sample. In other words, the observational efficiency is low. As was described above, a high voltage is applied to the sample; however, if a high voltage is abruptly applied, the sample will be damaged, and this damage may lead to failure in some cases. Accordingly, the elevation of the voltage applied to the sample is accomplished over a period of time in order to avoid damaging the sample.
These problems are even more severe in cases where secondary electrons are used for the observational electron beam than they are in cases where reflected electrons are used for the observational electron beam. The reason for this is as follows: generally, while reflected electrons are emitted from the sample in one direction, secondary electrons are emitted from the sample in an isotropic manner. Accordingly, in the case of secondary electrons, it is necessary to increase the quantity of secondary electrons drawn from the sample toward the cathode lens, i. e., the so-called yield, in order to improve the precision (S/N) of observation. Consequently, the electric field formed between the sample and the first electrode of the cathode lens must be correspondingly strengthened. As a result, a high voltage must be applied to the sample and to the illumination lenses, cathode lens and image-focusing lenses consisting of einzel lenses, thus fostering the two problems mentioned above.
Here, in cases where a method in which the yield of secondary electrons is increased by increasing the internal diameter of the aperture diaphragm is adopted in order to improve the precision of observation by means of secondary electrons, instead of adopting a method in which the electric field between the sample and the first electrode is strengthened as described above, a separate problem arises in place of the above-mentioned problems: namely, the aberration becomes worse, so that the resolution drops.
Accordingly, the object of the present invention is to provide an electron-optical system which has a low cost and a high observational efficiency while maintaining the overall electron path length and quality of observation, such as yield of observational electrons and resolution, etc.
The present invention was devised in order to achieve the above-mentioned object. Specifically, with symbols appearing in the attached figures noted in parentheses, the present invention is an electron-optical system which is characterized by the fact that in an electron-optical system which is equipped with an irradiation means that irradiates the surface of a sample (7) with an irradiating electron beam (S), and an observation means that focuses an observational electron beam (K) emitted from the surface of the sample (7) as an image on an electron beam detection means (11), and in which potential difference that accelerates the observational electron beam (K) is created between the surface of the sample (7) and the electrode (6a) of the observation means that is positioned closest to the surface of the sample (7), the electrode (6a) of the observation means that is positioned closest to the surface of the sample (7) is biased to a positive potential with respect to the ground potential.
Furthermore, the present invention is an electron-optical system which is characterized by the fact that in an electron-optical system in which an irradiating electron beam (S) generated from an irradiating beam source (1) is caused to be incident on the beam separator (4) via an illumination optical system (2, 3), the irradiating electron beam (S) passing through the beam separator (4) is caused to be incident on the surface of the sample (7) via an objective optical system (6), an observational electron beam (K) emitted from the surface of the sample (7) is caused to be incident on the beam separator (4) via the objective optical system (6), the observational electron beam (K) is directed by the beam separator (4) in a direction that differs from the direction leading to the irradiating beam source (1), the observational electron beam (K) that has passed through the beam separator (4) is caused to be incident on an electron beam detection means (11) via an image-focusing optical system (8, 10), and a potential difference that accelerates the observational electron beam (K) is created between the surface of the sample (7) and the electrode (6a) of the objective optical system (6) that is positioned closest to the surface of the sample (7), the electrode (6a) of the objective optical system (6) that is positioned closest to the surface of the sample (7) is biased to a positive potential with respect to the ground potential.